A radar system transmits a signal and receives its echo. By processing the echo signal, the radar system is able to detect objects, and to estimate the distances, velocities, and directions associated with the objects. Historically, a pulsed radar is used in military applications, where targets of interest are typically far away from the radar system. The pulsed radar emits short pulses, and in the silent period receives the echo signals. The transmitter of the pulsed radar system is turned off before the measurement starts. However, in many civilian applications, such as automotive radar, wireless gesture recognition, vital sign monitoring, and other monitoring implementations, the objects of interest are usually close to the radar. Due to the short round-trip-delay (RTD) of the desired reflection signal, a pulsed radar doesn't work as well at close range. Instead of a pulsed radar, a frequency-modulated continuous wave or waveform (FMCW) radar is used for short distances.
In FMCW radar, the transmission signal is modulated in frequency (or in phase) and differences in phase or frequency between the transmitted signal and a received signal are used to measure distance to the object from which the transmitted signal is reflected. A linear frequency modulated (LFM) waveform can be used, whose instantaneous frequency linearly increases or decreases over time. With the change in frequency being linear over a wide range, then the distance can be determined by a frequency comparison, with the frequency difference being proportional to the distance.
FIG. 1 is a block diagram of a typical FMCW radar. A waveform generator 102 generates a radar probing waveform, such as a LFM waveform (i.e., chirp). The generated waveform is amplified by a power amplifier (PA) 103, and then transmitted from the transmitter antenna 104 repeatedly and periodically. At the same time, the signal received by the receiver antenna 114 is first amplified by a low-noise amplifier (LNA) 113, and then mixed at mixer 106 with a replica of the transmitted waveform. A low-pass filter (LPF) 107 and a digital-to-analog (ADC) 112 are then applied to the output of mixer 106.
When LFM is used, the signal reflected from an object can be modeled as a sinusoid at the mixer output, whose frequency is proportional to the RTD. An ideal waveform for transmission in a FMCW radar can be taken to be a signal, s(t):s(t)=ej2π(fct+0.5γt2),  (1)with fc and γ being the center frequency for the waveform and the chirp rate, respectively. A chirp, which can be referred to as a sweep signal, is a signal in which the frequency increases or decreases with time. The chirp rate is the rate of change in the chirp. The instantaneous frequency for s(t) is given as fc+γt, linearly increasing over time t. The received signal, as a reflected signal from an object to which the transmitted signal is incident, can be modeled as x(t):x(t)=βs(t−τ)=βej2π(fc(t−τ)+0.5γ(t−τ)2),  (2)with β and τ being amplitude and delay, respectively. The signals s(t) and x(t) can be combined at mixer 106, for example, having output, y(t):y(t)=x*(t)s(t)=βej2π(fcτ−0.5γτ2)ej2πγtτ,  (3)which is a sinusoid over t. By applying a fast Fourier transform (FFT) to the output of ADC 112, for example, the object can be detected, the associated delay τ can be estimated, and an associated distance to the object can be generated.
In operation, a FMCW radar repeats transmission of the LFM waveform continuously or with a small gap between periods. Therefore, in FMCW radar, the transmitter and receiver operate concurrently. Because of this concurrent operation, the transmitted signal can leak into the receive channel from the transmitter (Tx) antenna to the receiver (Rx) antenna directly (without reflection), or even within the radio frequency (RF) circuit. This Tx-to-Rx leakage signal can be much stronger than the desired reflection signal, and can cause severe problems. First, it can generate “ghost” targets, which cause false detections. In such cases, the leakage signal acts as a representation that the transmitted probe had been reflected from an object. Since this is a detection of a leakage signal, the detected signal is not provided by an actual object; hence the detection is from a “ghost” target. Second, it can mask true targets, causing missing detection. Even worse, it can saturate components of the receiver, such as an analog-to-digital converter (ADC), and totally disable the radar system.
Therefore, it is a critical and challenging task to cancel or reduce the Tx-to-Rx leakage in FMCW radar or otherwise account for the Tx-to-Rx leakage. Many methods have been presented in the literature. Some methods perform the Tx-to-Rx leakage cancellation in the analog domain, whose performance is usually limited. A reported method performs leakage cancellation in the digital domain, which, however, cannot solve the receiver saturation problem. Some proposed methods consider a mixed leakage cancellation scheme, which estimates the leakage signal in the digital domain and cancels it in the analog domain. However, in such a mixed leakage cancellation scheme, the leakage cancellation step is carried out at radio frequency (RF) using additional mixers, which scheme increases the complexity and cost of a radar transceiver.